This invention relates to a process for making a printed wiring board which comprises preparing a printed wiring board utilizing an electrophotographic method using a material to be developed having a metal conductive layer and a photoconductive layer in this order on at least one surface of an insulating substrate and a preparation device thereof.
A method for making a printed wiring board is roughly classified into two methods of the subtractive method and the additive method. The subtractive method is a method of forming a resist layer on a laminated board to which a conductive layer such as copper, etc. is provided on an insulating substrate, and removing the conductive layer not covered by the resist layer by etching. The additive method is a method of forming a conductive layer only at a wiring pattern portion on an insulating substrate. In addition the above, it has been proposed a wiring transfer method in which a resist image is provided by a plate resist on a conductive substrate, metal plating is applied to the conductive substrate other than the resist image to form a metal wiring pattern, and then, the resist image is removed, only the metal wiring pattern is transferred to the insulating material.
Moreover, according to the recent trend of making light, thin, short and compact, or variety of electronic devices, in a printed wiring board, it is required to make it with high density and within a short period of time. Also, application of an electrophotographic material as a resist material has been investigated. Heretofore, in an electrophotographic material to be used in an electronic photography lithographic printing plate, etc., drawing an image at an infrared region has been already carried out, and it has been practically carried out that an image data are directly sent from a computer by a scanning exposure of a laser beam without using a photomask to form an image with a high density.
Also, not only the above-mentioned scanning exposure, but also a light-sensitive electrophotographic printing material is high sensitivity in a surface exposure using a photomask, so that it is not necessary to use the conventionally used expensive ultra-high pressure or high-pressure mercury lamp for printing a resist or an expensive UV exposure device such as a metal halide lamp, etc., and there is a merit of capable of using a simple and easy, and cheap visible light exposure device.
Also, in a photopolymer which is the conventional resist, an exposed portion of a photopolymer layer provided with a certain film thickness is cured or solubilized to form a wiring pattern. However, in this case, if light to be irradiated through a photomask is scattered light, light is spread in the photopolymer-layer, and in this case, a line width of an origin of the photomask cannot be reproduced accurately. Also, when dry resist film is used, resolution becomes worse due to spread of light still more in a scattered light since a polyester film exists between the photomask and the resist. This phenomenon becomes more significant when a wiring pattern becomes finer, and thus, light to be irradiated is required to be parallel light. As an exposure device therefor, there are a device in which light is tried to be parallel light by elongating the distance between a light source and an exposure surface, or a device in which a parabolic mirror is provided at the back surface of a light source and a light buffle is provided at an exposure surface side of the light source so that only the parallel light reflected from the back surface is irradiated to the exposure surface or the like. In either of the method, not only the device becomes complicated and a large-sized, but also utilization factor of light is lowered. On the other hand, as is clear from the explanation mentioned below, in the resist using the electrophotographic method, presence or absence of electric charge on the surface of a photoconductive layer contributes to resolution so that it is not so required to consider lowering in resolution by spreading light in the photoconductive layer. Accordingly, as an exposure device, that using a scattered light can be used and there is a merit of constituting an inexpensive and a small sized exposure system.
In the preparation method of a resist layer utilizing an electrophotographic method, a photoconductive layer is provided on a metal conductive layer of an insulating substrate on which a metal conductive layer is provided on the insulating substrate, and after uniformly charging the surface of the photoconductive layer in a dark place, an electrostatic latent image corresponding to a wiring pattern is formed by exposure. The electrostatic latent image is subjected to toner developing treatment and fixing, and by using the toner image as a resist, the photoconductive layer of the toner image unformed portion is dissolved and removed to prepare a resist image of a metal conductive layer comprising the toner image and the photoconductive layer. Dissolution and removal of the unnecessary portion of the metal conductive layer and the subsequent preparation step of the printed wiring board can be carried out in the same manner as in the conventional procedure.
A method of toner developing treatment can be roughly classified to a positive development method using a toner having a charge in an opposite polarity to an electrostatic latent image by exposing a non-image portion, and a reverse development method using a toner having a charge with the same polarity as that of an electrostatic latent image by exposing an image portion.
The preparation method of a printed wiring board by the electrophotographic method using the positive development method has been proposed in Japanese Patent Publication No. 35518/1989, and the preparation method of a printed wiring board by the electrophotographic method using the reverse development method has been proposed in Japanese Provisional Patent Publication No. 112627/1994.
At present, in a progress of high density preparation of a printed wiring board, a printed wiring board having a lot of conductive fine pores-called through holes provided to respective substrate with a multi-layered structure has been prepared. In this case, circuits formed on the both surfaces of the substrate are connected by the through holes. Thus, not only precision of the circuit itself formed on the substrate but also precision of formed positions of the circuits on the both surfaces relative to the through holes as a standard, i.e., positional precision prior to formation of the resist film are required.
Also, at the time of forming a wiring pattern, it is important to completely protect the metal conductive layer in the through hole by using a resist. That is, in the electrophotographic method, in the toner developing treatment step, it is necessary to form a toner image throughout the whole surfaces in the through holes.
When the electrophotographic method by the reverse developing method is used, procedure is carried out that the surface of a photoconductive layer is charged in a dark place and forming an electrostatic latent image is formed by vanishing a charge at the exposed portion by exposing an image portion, and then, fine toner particles having the same polarity as those of the charged electric charge are adhered to the exposed portion, i.e., to a portion at which the electric charge is vanished. At this time, a conductive member called a developing electrode is provided opposed to the surface of an electrostatic latent image, and a bias voltage with the same polarity is applied to the electrode whereby adhesion of the toner particles to the exposed portion is promoted and a toner image with a high image quality can be obtained.
A toner attached amount at the toner developing treatment is markedly dependent on a toner concentration, a developing time as well as the state of an electric field at around the exposed portion of a material to be developed. That is, as the electric field is stronger, adhesion of the toner can be carried out rapidly, and accordingly, the toner attached amount with a limited developing time becomes much.
The state of an electric field is regulated by the constitution of a developing electrode, positional relationship with a substrate, a bias voltage applied to the developing electrode and a surface potential of a substrate electrostatic latent image, etc. The electric field inside of the through hole becomes markedly weak as compared with the electric field at the surface of the substrate since the distance from the developing electrode and the distance of an unexposed portion of the substrate surface. Thus, a rate of toner attachment in the through hole becomes late as compared with the substrate surface, and as a result, the toner attached amount becomes a little. This tendency becomes particularly remarkable when a ratio of a substrate thickness and a diameter of the through hole, i.e., an aspect ratio of the through hole becomes high.
It is possible to increase an amount of the attached toner in the through hole by elongating the development time or thickening a toner density. However, it causes at the same time that the toner-attached amount to the wiring image at the both surfaces of the substrate is unduly increased. In a fine wiring pattern, due to excessive attachment of the toner, toner images of wiring adjacent to each other are fused, and as a result, to cause shortage between wiring. Accordingly, in order to prepare a printed wiring board having through holes in a good state, it must be increased an attached amount of the toner in the through holes without lowering resolution of a wiring image on the substrate surface.
If a sufficient amount of toner-attachment cannot be carried out in a through hole with a high aspect ratio, a part of the metal conductive layer in the through hole is removed by failure of the resist of the toner image at the time of removal of the photoconductive layer after toner developing treatment and subsequent removal of the metal conductive layer. In such a case, open circuit at the both surfaces of the substrate occurs or, even when there is an open circuit, there is a possibility of causing failure due to fatigue of an incompletely covered metal conductive layer in the through hole by heat applied to at the time of soldering, by various used circumstances of the substrate, by a heat cycle applied thereto during transportation, or by heating and cooling at repeated uses. This finally causes a problem of occurring breakage of the metal conductive layer, i.e., open circuit.
A method of preparing a resist layer to the both surfaces of a substrate utilizing the electrophotographic method has been disclosed in Japanese Provisional Patent Publication No. 224541/1994, and it is possible to form toner images on the both surfaces by simultaneously developing both surfaces of a laminated board in which a metal conductive layer and a photoconductive layer are provided on the both surfaces of an insulating substrate. Accordingly, it is sufficient to form an electrostatic latent image corresponding to a desired wiring pattern on at least both surfaces of the photoconductive layers. However, when a printed wiring board having a through hole is prepared by the electrophotographic method, as in the case where it is prepared by the method other than the electrophotographic method, it is required not only to protect the metal conductive layer in the through hole by a resist as a matter of course, but also to ensure precision of circuits provided on both surfaces or precision of their formed positions as well as a positional relationship of an electrostatic latent image itself and a static charge potential necessary at the said position.
An object of the present invention is to prepare a printed wiring board by utilizing the electrophotographic method. That is, for preparing not only a material to be developed having a metal conductive layer and a photoconductive layer on one surface of an insulating substrate, but also a printed wiring board having circuits on the both surfaces of a substrate from a material to be developed having at least a metal conductive layer and a photoconductive layer on the both surfaces of an insulating substrate, it is a technical task to provide a process for making a printed wiring board which can particularly ensure a positional relationship of an electrostatic latent image itself and a static charge potential necessary at the said position, is capable of forming a uniform and good image without any stain at the both surfaces, and when there are through holes, resist can be completely carried out by attachment of a toner at the inside of the through hole, open circuit is never caused and stable through holes having durability can be formed, and to provide a preparation device of the same.
A method for preparing a printed wiring board of the present invention comprises placing a material to be developed having at least a metal conductive layer and a photoconductive layer on both surfaces of an insulating substrate in this order on a platen to apply thereto alignment and static charge, then exposing one of the photoconductive layer of the material to be developed to provide an electrostatic latent image, reversing the material to be developed to apply thereto alignment and static charge again, exposing the photoconductive layer surface which is the opposite surface of the electrostatic latent image-formed surface of the material to be developed to form an electrostatic latent image, and subjecting to toner developing treatment.
Also, a method for preparing a printed wiring board of the present invention comprises statically charging photoconductive layers at the both surfaces of a material to be developed having at least a metal conductive layer and a photoconductive layer on both surfaces of an insulating substrate in this order, subjecting to alignment to a predetermined position, exposing the material to provide an electrostatic latent image on one photoconductive layer surface of the material to be developed, reversing said material to be developed to apply thereto alignment to a predetermined position, exposing the photoconductive layer surface which is the opposite surface of the electrostatic latent image-formed surface of the material to be developed to form an electrostatic latent image, and subjecting to toner developing treatment.
Also, a method for preparing a printed wiring board of the present invention comprises statically charging photoconductive layers at the both surfaces of a material to be developed having at least a metal conductive layer and a photoconductive layer on both surfaces of an insulating substrate in this order, exposing the material to provide an electrostatic latent image on one photoconductive layer surface of the material to be developed, subjecting to toner developing treatment to form a toner image, statically charging again both surfaces of the material to be developed, exposing the photoconductive layer surface which is the opposite surface of the toner image-formed surface of the material to be developed to form an electrostatic latent image, and subjecting to toner developing treatment.
Also, a method for preparing a printed wiring board of the present invention comprises providing development electrodes which can apply voltages between the above-mentioned metal conductive layers on the both surfaces of the substrate opposed to each other at the both sides of the substrate, statically charging photoconductive layers at the both surfaces of the above-mentioned material to be developed, exposing the material to be developed to form an electrostatic latent image on one of the photoconductive layer surfaces, and subjecting to toner developing treatment of the material to be developed by setting a bias voltage of the electrode which is so provided as opposing to the surface of the photoconductive layer on which no electrostatic latent image is formed to substantially 0 V, whereby good toner image can be formed and preparation of good printed wiring board can be accomplished.
Also, a method for preparing a printed wiring board of the present invention comprises statically charging photoconductive layers at the both surfaces of a material to be developed having at least a metal conductive layer and a photoconductive layer on both surfaces of an insulating substrate in this order, exposing the material to provide electrostatic latent images on the photoconductive layer surfaces at the both surfaces of the material to be developed, and subjecting to toner developing treatment by development electrodes at the respective surface to which at least two different bias voltages can be applied, respectively, to form a toner image.
Also, the above-mentioned material to be developed is not only a material having the constitution that at least a metal conductive layer and a photoconductive layer on both surfaces of an insulating substrate in this order, but also a material prepared by making a through hole(s) to a laminated board in which first metal conductive layers are provided on the both surfaces of an insulating substrate, subjecting to metal plating treatment of the laminated board to provide second metal conductive layers at the inside of the through hole(s) and on the surface of the laminated board, and then, forming photoconductive layers on the second metal conductive layers. By employing the latter material, required statically charging potential can be ensured, and uniform and good image can be formed on the both surfaces without any stain, resist with attachment of a toner at the inside of the through hole(s) can be completely carried out, whereby preparation of a printed wiring board which does not causes open circuit and is capable of forming durable and stable through hole(s) can be accomplished.
Also, the printed wiring board according to the method as mentioned above can be prepared by a preparation device which comprises a platen on which a material to be developed having a metal conductive layer and a photoconductive layer on at least one surface of an insulating substrate in this order is to be placed thereon, a supplying means of the material to be developed for placing the material to be developed on the platen, a statically charging means for statically charging at least the surface at which the photoconductive layer is provided of the material to be developed, a registration means for subjecting the material to be developed on the platen to alignment at a predetermined position, an exposure means for forming an electrostatic latent image on the statically charged photoconductive layer surface, and a reversing means of the material to be developed which is capable of reversing the material to be developed at least one surface of which is formed a static charge latent image.
Also, a preparation device of the printed wiring board according to the present invention comprises a platen on which a material to be developed having at least a metal conductive layer and a photoconductive layer on both surfaces of an insulating substrate in this order is to be placed thereon, a supplying means of the material to be developed for placing the material to be developed on the platen, a statically charging means for statically charging the photoconductive layers on the both surfaces of the material to be developed, a registration means for subjecting the material to be developed on the platen to alignment at a predetermined position, an exposure means for forming an electrostatic latent image on the statically charged photoconductive layer surface, and a reversing means of the material to be developed which is capable of reversing the material to be developed at least one surface of which is formed a static charge latent image.
Also, in the above-mentioned preparation devices of the printed wiring board, by having a grounding means which contacts with a side surface of the material to be developed placed on the platen, stable static charging can be carried out and static charging without disturbing uniform charged state on the both surfaces of the material to be developed can be carried out.
Also, in the above-mentioned preparation devices of the printed wiring board, if the statically charging means is a roll statically charging means, a material to be developed which has a metal conductive layer and a photoconductive layer in this order on the both surfaces of an insulating substrate which is a material to be statically charged can be conveyed and the both surfaces can be simultaneously statically charged.
Also, even when the material to be developed has the above-mentioned through hole(s), a printed wiring board in which toner is thoroughly adhered to the inside of the through hole(s) without causing breakage of a wire can be produced when a toner image is formed by the reverse development method under a suitable bias voltage application without generating unnecessary static charge in the through hole(s).
Also, in the above-mentioned preparation devices of the printed wiring board, if the device has a surface potential measuring means for measuring the surface potential of the material to be developed which is statically charged, inconvenience of the photoconductive layer of the material to be developed or a statically charging means can be known by measuring the surface potential of the material to be developed which is statically charged.
Also, the statically charged material to be developed is transferred under the surface potential measuring means, positional relationship between the surface potential measuring means and the statically charged material to be developed can be judged by the measured potential.
Also, in the preparation method of the above-mentioned printed wiring board, by measuring the surface potential of the statically charged surface, alignment of the statically charged material to be developed can be carried out without contacting them from the changed state of the measured potentials by the positional relationship between the surface potential measuring means and the statically charged material to be developed.
Also, in the preparation method of the above-mentioned printed wiring board, if the material to be developed has a through hole(s) as mentioned above, potential difference can be confirmed when the through hole(s) portion is passed under the surface potential measuring means so that highly precision alignment of the statically charged material to be developed can be realized by measuring the surface potentials at the through holes and near to the same and based on the positional relationships between the group of through holes.
Also, by making the above-mentioned exposure method a scanning exposure system using a light emitting diode array, the data from a computer can be directly used for exposure whereby the process can be simplified. And yet, it uses a light emitting diode array so that a complex optical system such as a polygon mirror or a fxcex8 lens is not used whereby preparation of a printed wiring board can be carried out with cheap, simple and easy optical system adjustment and with good precision.